You don't understand. We don't correlate intelligence with hard work.
We correlate intelligence with having to survive in hostile (cold) climate.
It's pretty self-explanatory that you have to think alot more if you want to survive in cold weather. Your housing needs to be better, it's more difficult to source food, you have to have warm clothes, need to keep your housing warm, you have to stock food in the summer and portion it in the winter, etc. etc.
This isn't something we pull out of our ass, it's based on science.